One of the reasons they've never done this...
They haven't really had prospects worth doing this with. Greene and Tork, I probably would have considered, but Greene got hurt right around the time you would pull the trigger on this. And Tork is a Boras guy.
Before that, Castellanos made some sense, but the CBT implications make that tricky. Because of the AAV nature of these type of deals, you don't really want to be entering into them when you are going to be fighting the tax, taking on a few extra million on your CBT Salary without upgrading the current roster.
Before that, Curtis Granderson signed an extension in 2008 after 2+ years.
And you don't do this with pitchers.